Broad band DUV, VUV long-working distance catadioptric imaging system

ABSTRACT

A high performance objective having very small central obscuration, an external pupil for apertureing and Fourier filtering, loose manufacturing tolerances, large numerical aperture, long working distance, and a large field of view is presented. The objective is preferably telecentric. The design is ideally suited for both broad-band bright-field and laser dark field imaging and inspection at wavelengths in the UV to VUV spectral range.

This application is a continuation of U.S. patent application Ser. No.11/033,218, filed Jan. 10, 2005, entitled “Broad Band DUV. VUVLong-Working Distance Catadioptric Imaging System.” inventors David R.Shafer, et al., which is based on U.S. patent application Ser. No.09/796,117, filed Feb. 28, 2001. entitled “Broad Band DUV. VUVLong-Working Distance Catadiontric Imaging System,” inventors David R.Shafer, et al., now U.S. Pat. No. 6,842,298, which claims the benefit ofU.S. Provisional Patent Application 60/231,761, entitled, “Broad BandDUV. VUV Long-Working Distance Catadioptric Imaging System,” filed onSep. 20, 2000, all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of optical imagingand more particularly to catadioptric optical systems used formicroscopic imaging, inspection, and lithography applications.

2. Description of the Related Art

Many optical and electronic systems exist to inspect surface featuresfor defects such as those on a partially fabricated integrated circuitor a photomask. Defects may take the form of particles randomlylocalized on the surface, scratches, process variations, and so forth.Such techniques and devices are well known in the art and are embodiedin various commercial products such as many of those available fromKLA-Tencor Corporation of San Jose, Calif., the assignee of the presentapplication.

Specialized optical systems are required to enable the imaging andinspection of surface defects, such as those found on semiconductorwafers and photomasks. Two prior inventions describe high numericalaperture (NA) catadioptric systems that can support this type ofimaging. These inventions are U.S. Pat. No. 5,717,518 to Shafer et al.,as shown in FIG. 1, and U.S. Pat. No. 6,064,517, also to Shafer et al.

U.S. Pat. No. 5,717,518 to Shafer et al. describes an apparatus capableof high NA, ultra broadband UV imaging. The '518 patent describes a 0.9NA system that can be used for broadband bright field and multiplewavelength dark-field imaging. It has ultra broad band chromaticcorrection using an achromatized field lens. This system can employ anachromatized field lens group to correct for secondary and higher orderlateral color. This type of design has several limitations including alimited working distance, central obscuration, internal pupil, and somerelatively tight manufacturing tolerances. The tight manufacturingtolerances mainly come from the balance of the large sphericalaberration generated from the catadioptric group.

U.S. patent application Ser. No. 09/349,036 filed on Jul. 7, 1999 toShafer et al., as shown in FIG. 2, describes a high NA optical apparatusthat has several advantages including a long-working distance, zerocentral obscuration, external pupil, and a relatively loosemanufacturing tolerance. The system presented in the '036 application isideally suited for use in long-working distance imaging applications,such as reticle inspection and lithography. This design is highlyapplicable for use in both 193 nm and 157 nm generation inspection andlithography applications. The design shown has a high degree ofchromatic correction using a single glass material. Further chromaticcorrection is possible using two glass materials. This design has a longoverall optical path, which in certain circumstances can adverselyaffect mechanical stability and manufacturing costs. Also, the opticalsystem has an unusual optical axis near perpendicular that can offermanufacturing challenges.

Other specialized catadioptric optical systems have been developed foruse in semiconductor lithography. These systems are designed to image aphotomask at a reduced magnification onto a resist coated wafer. U.S.Pat. No. 5,052,763 to Singh et al. discloses a catadioptric opticalsystem capable of high NA imaging. The '763 design creates asubstantially flat image field over the large areas required forsemiconductor lithography by having an input optical system with acurved field, a catadioptric relay system, and an output optical systemto correct for the field curvature and some monochromatic aberrations.This design has several limitations including a limited workingdistance, internal pupil, narrow bandwidth, an internal beamsplitter,and tight manufacturing tolerances.

It is therefore an object of the present invention to provide anapparatus that has a long working distance between the optical systemand the surface being inspected, a high numerical aperture, and smallsize.

It is also an object of the present invention to provide an apparatusthat has a high degree of chromatic correction using a single glassmaterial, where further chromatic correction can be achieved using atleast one additional glass material, thereby making the apparatus suitedfor use at wavelengths in the deep UV and vacuum UV ranges.

It is still another object of the present invention to provide anapparatus with an external pupil plane to support apertureing andFourier filtering.

It is another object of the present invention to provide an apparatushaving relatively loose tolerances enabling manufacture for a reasonablecost.

It is another object of the present invention to provide an apparatusthat has excellent image quality and a high degree of chromaticcorrection without the requirement to use aspherics, diffractive optics,beam splitters, or turning mirrors.

It is a further object of the present invention to provide an apparatusthat is suited to support microscopic imaging and inspectionapplications at wavelengths in the UV to VUV spectral range.

It is another object of the present invention to provide an apparatus tosupport both broadband bright field and laser dark field imaging andinspection.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a catadioptricobjective having correction of image aberrations, chromatic variation ofimage aberrations, longitudinal (axial) color and lateral color,including residual (secondary and higher order) lateral color correctionover a broad spectral range.

In previous systems, the order of the components, proceeding outwardsfrom the high NA image formed by the system, is as follows: catadioptriccavity, intermediate image and field lens, focusing lens group. Thepresent system reverses this previously employed ordering. The high NAimage is next to the focusing lens group, then an intermediate image andfield lens, then the catadioptric cavity. This reordering of componentsprovides a relatively long free working distance around the high NAimage as compared to previous systems.

In the previous component ordering scheme, the working distance is keptvery short in order to minimize the central obscuration of the system.Reversing that order provides a long working distance while keeping avery small obscuration, and also retains color correction design aspectsand characteristics found in designs using the previous componentordering scheme.

In addition to color correction, the present invention also includes anobjective that can be used as microscope or as micro-lithography opticswith a large numerical aperture, long working distance, and a largefield of view. The objective is preferably telecentric and is a highperformance objective with very small central obscuration, an externalpupil for apertureing and Fourier filtering, and relatively loosemanufacturing tolerances. The present invention is suited to bothbroad-band bright-field and laser dark field imaging and inspection atwavelengths below 350 nm.

These and other objects and advantages of the present invention willbecome apparent to those skilled in the art from the following detaileddescription of the invention and the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a previous catadioptric imaging design;

FIG. 2 is another previous catadioptric imaging design;

FIG. 3 illustrates an embodiment of the present invention having a 0.80numerical aperture and a 7.5 mm working distance;

FIG. 4 is an alternate embodiment of the current invention employing a193 nm optical design;

FIG. 5 is a further embodiment of the current invention with an emphasison loose tolerances;

FIG. 6 illustrates an alternate embodiment of the current inventionhaving a 0.85 numerical aperture for a 157 nm optical design;

FIG. 7 presents a conceptual embodiment of the current invention; and

FIG. 8 illustrates a conceptual view of a laser dark-field inspectionapparatus.

DETAILED DESCRIPTION OF THE INVENTION

This new configuration has several advantages over certain priorsystems, such as that shown in U.S. Pat. No. 5,717,518. The presentsystem has a long working distance, an external pupil for apertureingand Fourier plane filtering, very small central obscuration, very smallsystem length, distortion correction, deep UV achromatism with just oneglass type, and loose manufacturing tolerances. These advantages resultfrom the novel arrangement of the Mangin mirrors and the presence of thetwo intermediate images in the system. Other advantages to thisconfiguration include compact size, single optical axis, absence ofaspherics, diffractive optics and strong curvature surfaces.

FIG. 3 illustrates the present design. The design of FIG. 3 has a 0.80numerical aperture (NA) and a working distance of 7.5 mm, for use withan un-narrowed 193.3 nm excimer laser. All elements shown therein areformed from silica. A simple two lens third subsystem, shown on the leftside of FIG. 3, enables collimation of the output of the catadioptriccavity. From FIG. 3, pupil plane 301 is to the left of the device. Lightenergy initially passes through lens 302, lens 303, and mangin mirrors304 and 305, wherein light is reflected and transmitted through a holein mangin mirror 305. Field lens group 306 and focussing lens group 307serve to focus the light energy to the surface of the specimen (notshown). Field lens group 306 comprises first lens 306 a, second lens 306b, third lens 306 c, and fourth lens 306 d. Focussing group 307comprises first focussing lens 307 a, second focussing lens 307 b, thirdfocussing lens 307 c, fourth focussing lens 307 d, fifth focussing lens307 e, and sixth focussing lens 307 f.

Correcting the various color aberrations using a single glass typepresents specialized concerns. Conventional designs usually use two orthree glass types to correct color aberrations. The present inventioncorrects color aberrations using a single glass type due to theconfiguration of lens and mirror power. In the very deep UV range, bothsilica and CaF₂ are quite dispersive, so even a narrow spectralbandwidth at very short wavelengths can require the correction of quitea few distinct color aberrations. These may include: primary andsecondary axial color, primary and secondary lateral color, chromaticvariation of spherical aberration, and chromatic variation of coma.

The present optical system uses the Schupmann lens principle and theOffner field lens to correct for the primary axial and lateral color andthe secondary axial color, as described in U.S. Pat. No. 5,717,518. Thebasic concept is to use the color generated by some lenses in acatadioptric cavity, with a small central obscuration to let light intoand out of the cavity, to correct for the color introduced by a strongpositive power focusing lens group. The present system uses one or morefield lenses near the intermediate image, or between the catadioptriccavity and the focusing lens group, to image the catadioptric cavityonto the focusing lens group.

There are few glass materials that can be used for optical systems inthe DUV-VUV spectral range. For a design intended for use near 193 nm,the lens material of choice is silica. For a similar design intended foruse near 157 nm, that lens material is CaF₂. At 157 nm wavelength, forexample, there is only CaF₂ as a reasonable material that does not havesevere problems with birefringence, water solubility, or mechanicalsoftness. Further chromatic correction can also be achieved usingadditional glass materials.

In principle, only one lens/mirror element is needed based on the colorcorrecting principles applicable to this design, and the other elementforming the catadioptric cavity could be a simple mirror. However, a twolens/mirror element arrangement offers advantages even if onelens/mirror element has little or no optical functional advantage over asimple mirror.

The present system has a nearly unit magnification catadioptric cavitythat can be fashioned in a variety of sizes relative to the size of thefocusing lens group. As the catadioptric cavity is made larger, thepower of the lens part of the lens/mirror elements required for colorcorrection of the system becomes weaker. This effect occurs because alarge weak power lens can have the same amount of color as a strongersmaller size lens.

With respect to the small amount of central obscuration, the twolens/mirror elements forming the catadioptric cavity can have centralholes to allow the light into and out of the cavity. A boundary isneeded around the rim of the holes to prevent aperturing of the lightand allow for optical polishing errors. This effectively increases theamount of obscuration.

FIG. 4 illustrates an alternate arrangement without a central hole ineither lens/mirror element. The reflective coating of each elementincludes a central hole. An extremely small boundary is required for acentral hole in the reflective coating, and such a hole minimizes theamount of obscuration. This is important because a large centralobscuration blocks low frequency information and reduces light level.The present design as shown in FIGS. 3 and 4 does not have a centralobscuration problem and permits utilization of low frequencyinformation.

The embodiment of FIG. 4 is a 193 nm catadioptric objective using asingle glass material. In operation, light energy entering from the leftand passing through an external pupil plane is focused by first lens orpositive lens 402. A second lens or positive field lens 403 ispositioned in front of the focus. Diverging light then proceeds to acatadioptric lens group formed by two Mangin mirrors 404 and 405. AMangin mirror is a refractive lens/mirror element with a reflective backsurface. The catadioptric group re-images the first image onto thesecond image near the second Mangin mirror 405 at unit magnification.The second field lens group 406 may optionally be placed near the secondimage. Field lens group 406 includes first, second, and third fieldlenses 406 a-c. From there the light proceeds to the final focusinglenses 407. The focusing lens group includes first, second, third,fourth, and fifth focussing lenses 407 a-e. This final focusing lensgroup 407 provides a relatively long working distance to the image.

This embodiment has a bandwidth of 1 nm with the central wavelength at193.30 nm. The numerical aperture is 0.8, while the working distance isgreater than 7 millimeters. The central obscuration is less than 5% indiameter at 0.8 NA. Even at 0.35 NA, the central obscuration is stillbelow 16% in diameter, which is equivalent to 2.5% of area obscuration.The surface data for the first embodiment is listed in Table 1.

TABLE 1 Surface data of a 193 nm catadioptric design with a 1 nmbandwidth as shown in FIG. 4 Radius of Surface # curvature ThicknessGlass OBJ Infinity Infinity Air STO Infinity 15.188841 Air 2 −81.6279083.5 Silica 3 −18.040685 22.449116 Air 4 18.74457 2 Silica 5 795.1375921.998104 Air 6 84.996662 5 Silica 7 40.302422 97.532362 Air 8 −78.5674765 Silica 9 −132.110046 −5 Reflector/ Silica 10 −78.567476 −97.532362 Air11 40.302422 −5 Silica 12 84.996662 5 Reflector/ Silica 13 40.30242297.532362 Air 14 −78.567476 5 Silica 15 −132.110046 14.180612 Air 1641.906043 2.999944 Silica 17 −19.645329 0.499948 Air 18 10.2065346.643053 Silica 19 6.314274 5.385248 Air 20 −6.571777 8.442713 Silica 21−11.608676 19.085531 Air 22 29.380754 2.999908 Silica 23 25.2886974.186877 Air 24 55.554188 6.84081 Silica 25 −51.735654 0.5 Air 2653.425082 5.141563 Silica 27 −275.827116 0.5 Air 28 27.209707 5.295973Silica 29 85.400041 0.5 Air 30 13.757522 6.782701 Silica 31 69.4644238.236734 Air IMA Infinity

The designs in these embodiments require certain trade-offs for variousfeatures of the system based on different magnifications of the focusinglens subsystem, the magnification of the catadioptric cavity, and thesize of the catadioptric cavity.

The diameter of the catadioptric cavity and the magnification of thefocusing lens group determine the obscuration of the system. Use of asmall magnification for the focusing lens group reduces the diameter ofthe intermediate image and keeps the obscuration due to the catadioptriccavity relatively small. If that is not done and the intermediate imagesize is large due to excessive magnification by the focusing lens group,then a larger diameter catadioptric cavity is necessary to have aparticular amount of obscuration. The design shown in FIG. 4 has arelatively small catadioptric cavity due to a good choice of themagnification of the focusing lens group.

The catadioptric cavity can be fashioned in a variety of sizes relativeto the size of the focusing lens group. As the catadioptric cavity ismade larger, the power of the lens part of the lens/mirror elementsrequired for color correction of the system becomes weaker. This effectoccurs because a large weak power lens can have the same amount of coloras a stronger smaller size lens.

Properly controlling the size and magnification of the catadioptriccavity allows the catadioptric cavity to correct for sphericalaberration. If the cavity magnification is near one-to-one, thespherical aberration of the cavity is minimized. By changing thecatadioptric cavity magnification and size, spherical aberration can begenerated to compensate for the spherical aberration of other systemcomponents.

Another effect of the combination of the sizes of elements of thecatadioptric cavity relative to the size of the focusing lens subsystemand the magnification of the focusing lens subsystem is the Petzvalcurvature correction of the system. The catadioptric cavity partlycorrects for the strong Petzval curvature of the focusing lenssubsystem. The smaller the catadioptric cavity is, the stronger itselements and the more completely the catadioptric cavity cancels thePetzval curvature of the focusing lens group. Cancellation of thePetzval curvature of the focusing lens group may alternately beaccomplished by use of several negative lenses in the focusing lenssubsystem. The relative amount of Petzval correction of the completesystem by the catadioptric cavity, compared with the amount of Petzvalcorrection by the negative lenses in the focusing lens subsystem, affectthe ability to perform certain unrelated design tasks.

In addition it is desirable to have an optical design that can be easilymanufactured with a reasonable cost. Typically this requires low opticaland mechanical tolerances and a small size. To reduce the total lengthof the optical system requires a short catadioptric cavity and a lowmagnification focusing lens group. It is possible to achieve a designwith, small obscuration, good monochroamtic aberration correction, goodcolor correction, small size, and low tolerances and by properlychoosing the size and magnification of the catadioptric cavity and thefocusing lens group.

The design is telecentric on the high NA image end, and also has anexternal entrance pupil. Ideally the pupil is located a relativelysignificant distance from the nearest lens to provide an accessibleFourier plane and allow for the insertion of a beam splitter. These twoconstraints on entrance and exit pupil positions of the system affectthe Petzval correction distribution within the system and the lateralcolor correction of the system. As has been described, multipleinterreactions occur within the present optical system, and thus theembodiments shown exhibit a delicate balance between the optimumcatadioptric cavity and the construction of the particular focusing lenssubsystem.

The present optical system design has an external pupil plane to supportapertureing and Fourier filtering. The system may optionally employ anaperture to control the NA of the imaging system. Use of such anaperture enables control of the resolution and depth of focus.

The present optical system also has reasonable tolerances so it can bemore easily manufactured. Certain previously known high NA, broadbandwidth systems include optical elements having very tight positionand thickness tolerances. These tight tolerances frequently make thedesign too expensive or even impossible to build in a productionenvironment.

The second embodiment of the current invention is shown in FIG. 5. Thedesign of FIG. 5 is a more complex version of the 193 nm optical designwith emphasis on loose tolerances. From FIG. 5, light energy passespupil plane 501 to first lens 502 and field lens group 503 to manginmirror 504 and lens 505. Light energy reflects between the mangin mirror504 and mangin mirror 506 before leaving through the hole in manginmirror 506. Light then passes through field lens group 507, includingfirst through fourth field lenses 507 a through 507 d, and focussinglens group 508, including first through sixth focusing lenses 508 athrough 508 f.

One major difference between the design of FIG. 4 and that of FIG. 5 isthe addition of a lens 505 inside the catadioptric cavity. The extralens 505 enables simple modification of higher-order aberrations andresults diminished requirements for the field lens group 507. Theresulting field lens group 507 exhibits decreased tolerance sensitivitycompared with previous designs. The result is that every element in thedesign shown in FIG. 5 can be decentered by ±5 microns, and less than0.07 waves r.m.s. of on-axis coma is introduced. The 15 mm-diameterpupil 501 is 25 mm from the first lens 502, thereby more readilyenabling Fourier filtering. The surface data of the design shown in FIG.5 is listed in Table 2.

TABLE 2 Surface data of a 193 nm catadioptric design with improvedtolerance as shown in FIG. 5 Radius of Surface # curvature ThicknessGlass OBJ Infinity Infinity Air STO Infinity 25 Air 2 91.85042 3.5Silica 3 −24.379754 15.135303 Air 4 −12.478841 2 Silica 5 −16.341363 0.5Air 6 20.417844 2 Silica 7 −619.526297 11.13089 Air 8 81.628617 5 Silica9 46.38262 10.124973 Air 10 135.620838 5 Silica 11 73.040816 73.826866Air 12 −125.932264 5 Silica 13 −164.108468 −5 Reflector/ Silica 14−125.932264 −73.826866 Air 15 73.040816 −5 Silica 16 135.620838−10.124973 Air 17 46.38262 −5 Silica 18 81.628617 5 Reflector/ Silica 1946.38262 10.124973 Air 20 135.620838 5 Silica 21 73.040816 73.826866 Air22 −125.932264 5 Silica 23 −164.108468 4.997981 Air 24 −7.2170977.368018 Silica 25 −8.200826 4.888611 Air 26 −5.753897 2.495664 Silica27 −7.26511 0.49962 Air 28 −116.880084 2 Silica 29 −41.098609 0.537963Air 30 21.822865 9.566301 Silica 31 19.261704 59.973849 Air 32674.092453 4.5 Silica 33 −107.489747 0.5 Air 34 27.666171 2.998671Silica 35 25.945185 6.495392 Air 36 74.468846 5.52337 Silica 37−174.813731 0.5 Air 38 39.024119 5.297476 Silica 39 119.056539 0.5 Air40 22.33152 5.527379 Silica 41 37.203165 0.5 Air 42 12.178229 6.998013Silica 43 20.810308 10.113383 Air IMA Infinity

The above 193 nm catadioptric optical systems can be furtherchromatically corrected by using calcium fluoride lens elements,especially by placing such lens elements in the field lens group. Thesecalcium fluoride lens elements achromatize the field lens group andfurther increase the optical bandwidth.

The third embodiment of the present invention is a 0.85 NA design for157 nm and is shown in FIG. 6. From FIG. 6, pupil plane 601 is to theleft of first lens 602 and field lens group 603. Light energy passingthrough first lens 602 and field lens group 603 is directed to manginmirror 604, lens 605, mangin mirror 606, lens 607, filed lens group 608,and focussing lens group 609. Field lens group 608 comprises firstthrough third lenses 608 a through 608 c and focussing lens group 609comprises first through sixth lenses 609 a through 609 f.

The field sizes for the 0.85 NA and the 0.386 NA modes of operation are0.264 mm diameter and 0.58 mm diameter, respectively. A +/−5 micronsdecenter of any element leaves the on-axis Strehl value above 0.80, andthe arrangement shown in FIG. 6 has been optimized for this decentering.Addition of compensation elements provides for variances inmanufacturing tolerances without appreciable degradation. The surfacedata for one example of the type of design presented in FIG. 6 is listedin Table 3.

TABLE 3 Surface data of a 157 nm catadioptric design with a 0.2 nmbandwidth for the embodiment of FIG. 6 Radius of Surface # curvatureThickness Glass OBJ Infinity Infinity Air STO Infinity 25 Air 233.695296 4 CAF2 3 −37.093413 18.962478 Air 4 −8.620183 2 CAF2 5−10.369405 0.5 Air 6 11.074443 2 CAF2 7 21.480782 3.99975 Air 878.490827 5 CAF2 9 47.787034 13.247601 Air 10 178.58276 5 CAF2 1172.427167 71.003404 Air 12 −73.55544 5 CAF2 13 −131.363604 −5 Reflector/Silica 14 −73.55544 −71.003404 Air 15 72.427167 −5 CAF2 16 178.58276−13.247601 Air 17 47.787034 −5 CAF2 18 78.490827 5 Reflector/ Silica 1947.787034 13.247601 Air 20 178.58276 5 CAF2 21 72.427167 71.003404 Air22 −73.55544 5 CAF2 23 −131.363604 4.999004 Air 24 −10.2578 5.565984CAF2 25 −10.504881 22.77639 Air 26 13.922164 2.498855 CAF2 27 13.06363614.648618 Air 28 176.911005 4 CAF2 29 −153.13093 6.591557 Air 3071.277073 4 CAF2 31 150.529695 58.60825 Air 32 158.344855 7.324261 CAF233 −105.293511 0.499719 Air 34 54.468603 7.222634 CAF2 35 330.201607 0.5Air 36 26.746267 7.815818 CAF2 37 46.758643 0.5 Air 38 13.8399578.581743 CAF2 39 15.918572 13.159604 Air IMA Infinity

The optical designs described in the previous embodiments enablephotomask and wafer imaging and inspection at wavelengths of 193 nm and157 nm. Designs optimized for other UV wavelengths such as 365, 351,266, 248, and 126 nm and shorter wavelengths as well as those supportingmultiple wavelengths can be obtained by those skilled in the art usingthe optical designs and approaches presented in these embodiments.

The previous embodiments of the current invention have a long workingdistance between the optics and the surface being inspected, a highnumerical aperture, and a small central obscuration. A long workingdistance is essential for certain applications such as photomaskinspection and laser dark-field inspection. Photomask inspectionrequires the working distance of the imaging system to be greater than 7mm because of the protective pellicle on the photomask. A long workingdistance is also desirable for performing inspections in a laserdark-field environment. Use of a long working distance enables directillumination from outside the objective of the surface being inspected.A high numerical aperture provides resolution imaging and collecting aslarge a solid angle as possible. Employing the design of the presentsystem enables numerical apertures of 0.8 with excellent performance. Anumerical aperture of 0.8 corresponds to collecting angles above thesurface from normal to 53 degrees.

An additional embodiment of the apparatus allows for photomaskinspection from the UV to VUV wavelenghts. This embodiment isillustrated in FIG. 7. The apparatus of FIG. 7 consists of illuminationoptics 101, a long working distance catadioptric imaging objective 102,image forming optics 103, and a detector 104. Catadioptric imagingdesigns using the design of FIG. 7 and employing a single glass materialare possible when using an illumination source having a bandwidth lessthan or equal to 1 nm. Designs using two glass materials are possiblewhen using an illumination source having a bandwidth greater than 1 nm.

The illumination source can be a variety of different types. Forexample, an un-narrowed excimer laser, a lamp with a bandpass filter, ora frequency converted laser can each produce light with a 1 nm bandwidthor less. An unfiltered lamp or lamp with a larger bandpass filter, suchas an excimer lamp or a deuterium lamp, are also available sources for abandwidth greater than 1 nm. It can be particularly difficult to find asuitable light source at 193 nm and 157 nm because there are few lightsources available at these wavelengths. The type of illumination can beeither transmitted or reflected light or both. The transmitted light forillumination system of the present design uses a condenser objective.The condenser objective does not require high optical quality becausethe objective is only used for illumination. The illumination forreflected light uses a beamsplitter and is implemented as typically donein a standard microscope.

The long working distance imaging objective for the design of FIG. 7 issimilar or identical to those described in FIGS. 4-6. The objective hasa long working distance, is highly corrected for all aberrations, andhas a large field of view. It is also advantageous for the objective tobe unobscured. The long working distance is provides clearance for theprotective pellicle included on the photomask. This pellicle istypically located about 6 mm above the mask surface, and performs thefunction of preventing dust and other contamination from reaching thephotomask surface. For this reason, the objective working distance idsdesigned to be greater than 6 mm so it will not interfere with thepellicle. The objective is also well corrected for aberrations over thebandwidth of the illumination source.

Most commonly available illumination sources have a bandwidth greaterthan the 1-2 pm bandwidth obtained from a standard type single materialall refractive objective design. The catadioptric designs in FIGS. 4-6address this problem. The objective is also capable of imaging over alarge field. Large fields and high data acquisition rates makeinspecting the photomask as fast as possible.

The image forming optics are corrected over the bandwidth of thecatadioptric imaging objective. These image forming optics also canachieve the various magnifications required by a photomask inspectionsystem. One technique for designing the image forming optics is to havethe image forming optics and the catadioptric objective fully correctedfor aberrations. Full correction for aberrations permits testing ofimage forming optics and separate testing of the catadioptric objective.Aberration correction may alternatively be shared between thecatadioptric objective and the image forming optics. Such an opticaldesign, while structurally simpler, can complicate the testing of theimage forming optics and the catadioptric objective.

The detector is a high speed detector capable of the high data ratesused for an inspection system. One detector applicable to the currentsystem is a single point diode type detector or an area type detectorsuch as a CCD or a CCD operating in the Time Delay and Integration.(TDI) mode. Such a detector should have a high quantum efficiency, lownoise, and a good Modulation Transfer Function (MTF).

An alternate embodiment uses the current optical system invention as anapparatus for laser dark-field wafer inspection below 266 nm. Thisembodiment is illustrated in FIG. 8. This apparatus consists ofillumination optics 801, a long working distance catadioptric imagingobjective 802, a Fourier filter or aperture 803 at the external pupilplane 804, image forming optics 805, and a detector 806. Catadioptricimaging designs incorporating the design of FIG. 8 using a single glassmaterial are possible when using an illumination source having abandwidth of 1 nm or less. Catadioptric imaging designs incorporatingthe design of FIG. 8 using two glass materials are possible when usingan illumination source with a bandwidth greater than 1 nm.

The types of illumination that can be used for this system are similarto those of the design presented in FIG. 7 for photomask inspection. Thepreferred source is a laser for directionality and brightness. Thepreferred type of laser-dark field illumination is direct illuminationof the wafer from outside the objective. Only light scattered from thewafer is collected by the catadioptric objective. The specularlyreflected beam is beyond the numerical aperture of the objective and isnot collected.

The long working distance imaging objective is similar to the imagingobjectives described in FIGS. 4-6. As with photomask inspection, it isdesirable for an objective used for laser directional dark-fieldinspection to have a long working distance, be highly corrected for allaberrations, have a large field of view, and an easily accessible pupilplane. It is also advantageous for the objective to have very smallobscuration. The long working distance makes it relatively simple todeliver laser energy to the wafer from outside the objective withoutinterfering with the operation of the imaging system. The objectiveshould also be well corrected for aberrations over the bandwidth of theillumination source. Most of the available illumination sources have abandwidth greater than the 1-2 pm bandwidth obtained from a standardtype of single material all refractive objective design. Thecatadioptric designs in embodiments 1-5 address this problem. Theobjective is also capable of imaging over a large field. Large fieldsand high data acquisition rates are essential to inspect the photomaskas fast as possible. The objective also has an easily accessible pupilplane to support Fourier filtering or aperturing. Fourier filtering canreduce the noise caused by repeating patterns on the wafer, therebyincreasing the signal-to-noise of defects on the surface.

The image forming optics are corrected over the bandwidth of thecatadioptric imaging objective and are also capable of the variousmagnifications required by a wafer inspection system. The image formingand the catadioptric objective can each be fully corrected foraberrations, enabling testing of image forming optics and thecatadioptric objective as separate units. Another technique is to shareaberration correction between the catadioptric objective and the imageforming optics. The optical design for this approach can be simpler, butit can complicate testing of the image forming optics and thecatadioptric objective.

The detector is preferably a high speed detector capable of the highdata rates used for an inspection system. One applicable detector is asingle point diode type detector or an area type detector such as a CCDor a CCD operating in the Time Delay and Integration (TDI) mode. Thisdetector has a high quantum efficiency, low noise, and a good ModulationTransfer Function (MTF).

The opaqueness of CMP layers in the DUV-VUV spectral range makes asystem using optical designs, such as the ones shown in FIG. 5 and FIG.6, well suited to finding surface defects and microscratches. Thepresent design is also well suited for use as a lithography lens or usedfor lithography simulation and can be used as a research and developmenttool for micro-electronic development. This design may be employed inbiological settings where a long working distance is needed between theoptics and the sample. The design is also applicable for wavelengths inthe range from visible to DUV to VUV. For this reason it is well suitedfor fluorescence measurements.

As with many DUV-VUV optical systems, it is often required to purge theoptical path with a high purity gas, or maintain the optical path in apartial vacuum. The main reasons for this are optical absorption anddamage. At short wavelengths, especially below 200 nm, many materialsare highly absorbing including oxygen, water, hydrocarbons, outgasingfrom epoxies and urethanes, and residues from cleaning solvents. Thesematerials, can absorb light and reduce transmission. Many materials canalso be photolitically deposited on surfaces. These surfaces then becomeabsorbing and have their transmission greatly reduced and even becomedamaged.

While the invention has been described in connection with specificembodiments thereof, it will be understood that the invention is capableof further modifications. This application is intended to cover anyvariations, uses or adaptations of the invention following, in general,the principles of the invention, and including such departures from thepresent disclosure as come within known and customary practice withinthe art to which the invention pertains.

1. An imaging system for imaging a specimen, comprising: a focusing lensgroup configured to collect light from said specimen; and a catadioptricgroup configured to correct chromatic aberration in light collected fromthe specimen using a plurality of mangin reflective elementssubstantially linearly aligned along a common axis; wherein saidfocusing lens group and said catadioptric group are configured toprovide light energy toward the specimen at a numerical aperture inexcess of approximately 0.8.
 2. The imaging system of claim 1, furthercomprising a field lens group configured to pass light energy betweensaid focusing lens group and said catadioptric group.
 3. The imagingsystem of claim 1, wherein said focusing lens group is separated fromsaid specimen by at least approximately 2 mm.
 4. The imaging system ofclaim 2, wherein at least one of said lens groups comprises a lens madeof silica.
 5. The imaging system of claim 2, wherein at least one ofsaid lens groups comprises a lens made of calcium fluoride.
 6. Acatadioptric optical system employed to inspect a specimen, comprising:a catadioptric group comprising a plurality of Mangin elementssubstantially linearly aligned along an axis; and image forming opticsbetween said catadioptric group and the specimen configured to receivelight energy from the specimen and pass the light energy to thecatadioptric group; wherein said catadioptric optical system exhibits anumerical aperture in excess of 0.8.
 7. The catadioptric optical systemof claim 6, wherein said catadioptric optical system provides chromaticcorrection over a range of wavelengths in the DUV or VUV range.
 8. Thecatadioptric optical system of claim 7, wherein said range ofwavelengths is at least 1 pm in width.
 9. The catadioptric opticalsystem of claim 7, wherein said range of wavelengths is at least 2 pm inwidth.
 10. The catadioptric optical system of claim 6, wherein saidcatadioptric optical system provides chromatic correction for higherorder chromatic aberration.
 11. The catadioptric optical system of claim7, wherein said catadioptric optical system additionally corrects forspherical aberration, coma and astigmatism.
 12. The catadioptric opticalsystem of claim 6, wherein said catadioptric optical system corrects forat least secondary longitudinal color and primary and secondary lateralcolor.
 13. The catadioptric optical system of claim 11, wherein saidcorrection is performed in the DUV or VUV range over a bandwidth of atleast 1 pm.
 14. An apparatus for imaging a semiconductor wafer,comprising: a focusing lens group of lenses aligned along a centralaxis; a catadioptric group comprising a plurality of mangin elementssubstantially linearly aligned along the central axis and configured toat least partially correct chromatic aberration; and a field lens groupof lenses located along an optical path between said focusing lens groupof lenses and said catadioptric group.
 15. The apparatus of claim 14,wherein said semiconductor wafer is separated from said apparatus by adistance sufficient to purge oxygen and moisture from a region separatedfrom said semiconductor wafer.
 16. The apparatus of claim 15, whereinsaid region is maintained in a vacuum.
 17. The apparatus of claim 14,wherein said system is used to image an object maintained in a moistureand oxygen free ambient environment.
 18. The apparatus of claim 17,wherein said ambient environment comprises a vacuum.
 19. The apparatusof claim 14, wherein the catadioptric group at least partially correctsfor spherical aberration.
 20. The imaging system of claim 11 wherein thecatadioptric group at least partially corrects for spherical aberration.